My invention relates generally to a fluid operated circuit for controlling the ascent and descent of the pistons of a pair of hydraulically operated cylinders of a dual post article lift assembly. More specifically, my invention relates to a fluid operated circuit for preventing the descent of such pistons when one of the movable article supporting structures associated with the cylinders is blocked, jammed or otherwise restrained from descending while the other is free to do so.
One type of prior art dual post article lift assembly is characterized by a hydraulically operated driven or drive cylinder containing a movable piston connected to a piston rod and hydraulically operated slave or idler cylinder also containing a movable piston connected to a piston rod. The drive and idler cylinders are hydraulically connected together for the transfer of hydraulic fluid therebetween such that upward and downward movements of the idler cylinder piston are dependent upon and controlled by similar movements of the drive cylinder piston. Raising and lowering of both pistons is accomplished by pumping hydraulic fluid into and draining hydraulic fluid from a lower end of the drive cylinder below the lowest attainable position of the drive cylinder piston. Article supporting structures including a pair of carriage arms are connected to each of the piston rods for lifting and lowering massive articles such as vehicles, for example, as the pistons are raised and lowered.
In some prior art dual post assemblies of this type, the article supporting structures are fixedly connected to the piston rods so that each of the structures always moves as the corresponding piston to which it is rigidly connected moves. However, because each of the supporting structures is operated independently of the other by a different one of the cylinders, problems have been encountered in maintaining proper alignment between them as they are lifted and lowered and especially when lowered. Failure to maintain adequate alignment of a vehicle on such a lift while it is lifted and lowered can obviously produce disasterous results. The problem has been dealt with in the case of dual post lifts adapted for below ground disposition by use of a rigid bar connected between the piston rods and/or between the supporting structures whereby neither of the pistons, rods or their attached article supporting structures can move unless the other does so. See, for example, the device disclosed in U.S. Pat. No. 1,982,936 issued to L. C. Stukenborg on Dec. 4, 1934. While such rigid attachment between the rods and/or their article supporting structures may be feasible for use with below ground dual post lift assemblies, the same is impractical for use with modern above-ground dual post lifts because the rigid interconnecting member tends to get in the way of the vehicle or other article being lifted.
To address this problem in above-ground lifts, resort has been had in the prior art to the use of a chain connected between the movable article supporting structures of each cylinder which is strung through a series of pulleys connected to the stationary housings covering the cylinders at levels above and below the highest and lowest attainable positions of article supporting carriage arms of the structures so that one structure can not move unless the other does so. While the chain can thus be strung so as to avoid interfering with the vehicle being lifted, it is necessary to string the same across a floor which forms the drive way for the vehicle between the two housings of the lift apparatus. This requires placing an elongated steel cover having beveled or tapered sides over the chain so as to permit passage of the wheels of the vehicle to be lifted thereover when approaching and leaving the lifting position. It is clearly desirable therefore to find satisfactory means for maintaining carriage arm alignment without using such a chain.
Another type of prior art above-ground dual post lift employs hydraulic cylinders whose pistons and attached piston rods are not fixedly connected to the lifting structure. Most modern lifts have a pulley connected to the upper end of each piston rod over which extends a chain connected between a lower end portion of the corresponding movable supporting structure and a base or support plate upon which the corresponding stationary cylinder housing is disposed. The attached drawing FIGURE shows such a prior art arrangement. As the piston rod of each cylinder is lifted and lowered by a given distance, the pulley associated therewith travels along the corresponding chain to lift and lower the corresponding article support structure by twice that distance. Accordingly, for a given height of maximum lift, the subject cylinders need only be about half as long as they would otherwise need to be in cases where the piston rods are fixedly connected directly to the movable supporting structure. In this type of lift, the movable article supporting structures ascend as the pistons ascend, but the pistons are free to descend independently of the supporting structure and may do so even though the supporting structures are locked, jammed, blocked or otherwise restrained from descending from an elevated position. Consequently, if one of the supporting structures is jammed at an elevated position while the other is free to descend when hydraulic fluid is drained from the drive cylinder, the carriage arms of the supporting structure can become seriously misaligned, sometimes with tragic results.
For many years it has been readily apparent that the problems of carriage arm misalignment in these above-ground dual post lift assemblies, particularly during descent, could probably best be solved by fully hydraulic means if a suitable fluid circuit for sensing and reacting to impending carriage arm misalignment could be devised. Unfortunately this problem has long plagued the prior art without a satisfactory solution.
By means of my invention, these and other difficulties encountered with above-ground, dual post lift assemblies have now been substantially overcome.